1. Field of the Invention
The present invention generally relates to an image forming apparatus including a recording head for ejecting recording liquid droplets.
2. Description of the Background Art
As for known image forming apparatuses, there are printers, facsimile machines, copiers, plotters, and multi-function machines. One example of such image forming apparatuses is a liquid jet recording apparatus (e.g., an inkjet recording apparatus). The liquid jet recording apparatus performs an image forming (also referred to as “recording”, “printing”, and the like) operation by using a recording head that ejects droplets of ink droplets onto a sheet. The liquid jet recording apparatus includes, for example, a serial type image forming apparatus that forms images by ejecting liquid droplets while moving the recording head in a main scanning direction, or a line type image forming apparatus that forms images by ejecting liquid droplets without moving the recording head.
It is to be noted that the term “image forming apparatus” includes apparatuses that form images by depositing ink to materials such as paper, string, fiber, fabric, leather, metal, plastic, glass, wood, ceramic, etc. Furthermore, the term “image formation” not only includes forming images that have a meaning (e.g., letters, shapes) on a medium, but also includes forming images having no particular meaning (e.g., patterns, liquid droplets arbitrarily deposited to a medium). Furthermore, the term “ink” not only includes so-called ink, but also includes any liquid that can be used to form images (e.g., recording liquid, fixing liquid). Furthermore, the term “sheet” not only includes papers, but also includes any materials onto which ink droplets can adhere (e.g., a sheet for an overhead projector, a fabric, a so-called recording medium or recording sheet, and the like).
As an example of such image forming apparatuses, Japanese Patent Registration No. 2897960 (Patent Document 1) discloses an image forming apparatus that transports a sheet adhering to a transport belt due to an electrostatic attraction force to ensure a flatness of the sheet and a high-accuracy in a distance between a recording head and the sheet because it is necessary to raise accuracy of a deposit position of liquid droplets on the sheet.
Japanese Patent Laid-Open Publication No. 2007-175979 (Patent Document 2) discloses an image forming apparatus including a sheet ejection/transport device so as to transport the sheet for ejection at a downstream side of a transport belt, which transports a sheet on which an image is formed, with respect to a sheet transport direction.
In a serial type image forming apparatus, sheets are intermittently transported by a transport belt. Therefore, it is better that a sheet ejection/transport device, which is disposed at a downstream side of the transport belt with respect to the sheet transport direction, is also driven intermittently in synchronization with the sheet transportation by the transport belt. In this case, if the intermittent transport amount by the sheet ejection/transport device is the same or less than the intermittent transport amount by the transport belt, the sheet transported from the transport belt may become bent, resulting in a sheet jam. Alternatively, the electrostatic attraction force may dissolve due to an uplifting of the transport belt or the sheet, resulting in a trouble in an image forming position. Consequently, it is set so that the intermittent transport amount by the sheet ejection/transport device is more than the intermittent transport amount by the transport belt to thereby maintain a tension on the sheet being transported and keep the sheet flat.
However, when the transport belt and the sheet ejection/transport device are driven with such a configuration, if the tension of the sheet by the sheet ejection/transport device is stronger than the holding force of the sheet (force for holding the sheet on the belt surface) by the transport belt, the sheet is not intermittently transported correctly against the recording head, resulting in an image degradation.